It is now well-known to convey magnetic toner from a toner supply hopper to a latent image development zone of an electrostatic copying machine by a magnetic transport roll.
Such a roll includes a non-magnetic shell or sleeve within which rotates a magnet roll. One form of such a magnet roll includes a plurality of elongated magnets of alternating polarity spaced about the circumference of the roll, parallel to the axis. As the magnet roll rotates in one direction, the bipolar magnetic toner rolls along the surface of the shell in the other direction under the magnetic influence of the constantly alternating polarities. Such a magnetic transport roll is shown in U.S. Pat. No. 3,455,276 granted to G. R. Anderson on July 15, 1969.
Conventional apparatus for conveying magnetic toner from a supply in a hopper to a magnetic transport roll is shown in FIG. 1, including a toner supply hopper 10 for holding a supply of magnetic toner 12. The magnetic toner 12 is fed through a discharge port 14 onto the surface of a magnetic transport roll, shown generally as 16.
The magnetic transport roll 16 includes a non-magnetic shell 18 within which is positioned a rotatable magnet roll 20 having elongated magnets of alternating polarities in its circumference. A feed path is provided by a control plate 22 from which it is attracted to the surface of the non-magnetic shell 18 by the magnetic flux density of the magnet roll 20.
Assuming that the magnet is rotated clockwise and the cylindrical shell 18 is fixed, the magnetic toner is drawn out from the discharge port 14 and then lies on the cylindrical shell 18. The magnetic toner 12 on the cylindrical shell 18 moves in a counter-clockwise direction. A doctor blade 24 determines the thickness of the layer of toner 12 being conveyed by the transport roll 16 to a development zone 26 where the toner is brought into contact with latent electrostatic images on a drum 28.
Magnetic toner 12 which has not adhered to the latent images in the apparatus depicted in FIG. 1 continues to advance in counterclockwise direction around the surface of the shell 18 under the influence of the magnet roll 20. A problem then arises as the unused toner accumulates at the discharge port 14 and the accumulation backs up against the counterclockwise flow of the toner.
FIG. 1 illustrates an example of such accumulation. At the start of operation an adequate amount of a toner 12 is drawn out of the toner supply hopper 10 onto the shell 18. Upon high speed rotation of the magnet roll 20 within 5 minutes the accumulation grows to the size b, within 10 minutes to the size c, and within 15 minutes to the size d. This phenomenon, not only results in a significant degradation in the quality of the image by toner backing up into the development zone 26, but also contaminates the adjacent equipment with excess toner escaping from the attraction zone of the magnet roll 20.